Crustal Anisotropy Measured Near the Calico Fault

We present preliminary results from a study of crustal anisotropy near the Calico Fault located in the Eastern California Shear zone. Shear wave splitting is used to approximate the in situ stress field and/or shear fabric near the fault and examine whether proximity to the main slip plane affects the observed anisotropy. 65 stations, a combination of L22 and 40T sensors, were deployed in a grid spanning 4 km across and 1.5 km along the Calico fault between June and November 2006 to explore seismic properties of a fault that has not experienced a major earthquake in hundreds to thousands of years. In this study we examine shallow crustal anisotropy determined using earthquakes that occur within the shear wave window, with an angle of incidence less than 45 degrees, during the experiment period. Splitting parameters are determined using an automated cross-correlation method that determines the fast direction and delay time for each station-event pair. Results show some spatial variation in shallow crustal anisotropy as the N30°W trending Calico fault is crossed from west to east. Fast directions rotate from parallel to the regional stress field, which is oriented approximately N30°E, towards north. Results also show significant scatter in fast directions and delay times within a region 1.5 km wide along the fault which supports a damage zone along the fault as found in previous studies.